SwiftPM: support for foreign C-focused build systems

[lukasa (Cory Benfield)]

@stackotter

Another issue is that currently, extensible build tools can include prebuilt binaries but iirc they only support Mac, which is pretty restrictive. This is definitely an issue unless autoconf can be easily built as a SwiftPM target.

This is definitely a restriction I want us to lift, but it requires coordination amongst the SSWG and the Swift Core Team to understand exactly how it is we want to structure the support.

I've spoken about the concrete problem elsewhere on the forum ([PITCH] Support for binary dependencies - #2 by lukasa and linked topics). This is a very solvable problem, but we do have to do some work to specify what this means and produce tooling that is capable of solving the issue.